B.5.15—Variable resistance

Variable resistance

A variable resistor lets the resistance in a circuit be changed. Increasing the resistance of a series variable resistor reduces the current for a fixed supply voltage. A rheostat normally uses two terminals to control current; a potentiometer uses three terminals as a potential divider.

Predict the circuit response

For a fixed supply, use $I=V/R_{total}$. If the variable resistance increases, total resistance increases and current decreases. In a series circuit, the potential difference across the variable resistor increases while the potential difference across a fixed series component decreases.

Sensor examples

An LDR has resistance that depends on incident light intensity. An NTC thermistor has lower resistance at higher temperature. These components allow a circuit to respond to its surroundings, but the resistance–stimulus relationship must be obtained from data or a stated model.

Common trap

Do not assume that “more resistance” means a larger current. First decide whether the supply voltage is fixed and whether the component is in series or parallel. For an internal-resistance investigation, changing a variable resistor is useful because it creates multiple $V$-$I$ data points.